Sensitive and High-Throughput Analysis of Volatile Organic Species of S, Se, Br, and I at Trace Levels in Water and Atmospheric Samples by Thermal Desorption Coupled to Gas Chromatography and Inductively Coupled Plasma Mass Spectrometry

Emissions of volatile organic sulfur (S), selenium (Se), bromine (Br), and iodine (I) species from aquatic ecosystems represent an important source of these elements into the atmosphere. Available methods to measure these species are either not sensitive enough or not automated, which hinder a full understanding of species distribution and production mechanisms. Here, we present a sensitive and high-throughput method for the simultaneous and comprehensive quantification of S, Se, Br, and I volatile organic species in atmospheric and aqueous samples using a preconcentration step onto sorbent tubes and subsequent analysis by thermal desorption coupled to gas chromatography and inductively coupled plasma mass spectrometry (TD-GC-ICP-MS). Selected commercially available sorbent tubes, consisting of mixed porous polymer and graphitized black carbon, offered the highest trapping capacity and lowest loss of species when stored at −20 °C for 28 days after sampling. After optimization of the TD-GC-ICP-MS method, absolute detection limits were better than 3.8 pg, 9.1 fg, 313 fg, and 50 fg, respectively, for S, Se, Br, and I species. As a proof of concept, the concentrations of target species were determined in aqueous and continuously collected atmospheric samples during a cruise in the Baltic and North Seas. Moreover, unknown S, Br, and I volatile species were detected in both aqueous and atmospheric samples demonstrating the full potential of the method.


Table of contents
. Properties of the sorbents used in the multi-bed sorbent tubes described in Table S2....... S5 Table S5. Conditioning programs applied to the various sorbent tubes using the TC-20 TM conditioner with the N2 flow set at 100 mL.min -  Table S1. Purity and suppliers of the volatile organic standards and gases used in this study. All suppliers are located in Switzerland.

Section A. Description and optimization of the TD unit and sorbent tubes
In the TD unit, species desorption and their transfer to the GC occurs in two consecutive desorption steps. In the first one, species are desorbed from the sorbent tube and swept onto the focusing trap, i.e., a thin tube made of quartz, containing a sorbent material in lower quantity than the sorbent tube itself, where a second cycle of sorption/desorption occurs. The TD optimization was conducted in two steps: (i) optimization of the temperature, time and N2 flow rate for the sorbent tube desorption and (ii) optimization of the desorption temperature and time, initial temperature of the focusing trap set prior the desorption, and heating rate for the focusing trap desorption step.
The sorbent tubes can be found at the following link: https://markes.com/shop/products/inert-steeltubes-uncapped. Sorbent tubes containing two or three sorbents are packed by mass to give approximately equal bed lengths for each sorbent with masses varying between 200 mg and 1000 mg per sorbent. A commercially available conditioner (TC-20, Markes International Limited) that can condition simultaneously 20 sorbent tubes was used for both the initial tubes conditioning following the manufacturer guidelines and afterwards for their reconditioning using a shorter program (Table   S3). Once conditioned, the sorbent tubes were closed with brass caps and stored in double plastic zip bags at room temperature. No contamination was detected after the initial conditioning or the reconditioning program.  Table S4. Properties of the sorbents used in the multi-bed sorbent tubes described in Table S2.  Table S5. Conditioning programs applied to the various sorbent tubes using the TC-20 TM conditioner with the N2 flow set at 100 mL.min -1 .

Initial conditioning Reconditioning
2hr (350°C) -4hr (380°C) a 30 min (380°C) a a manufacturer guideline, b own optimized reconditioning program determined after loading 5 ng of each species on sorbent tubes and looking for residuals after heating at 330°C for 30, 45, 60, 90 and 120 min.

Section B. Calibration and determination of detection limits
The calibrations for the volatile organic species were measured using different split ratios of the TD unit for the five sorbent tubes. This approach was found more accurate and reproducible than preparing diluted solutions for which losses of volatile species occur. The split ratio 3.8:1 was used as reference and the amounts of volatile species injected (minj) in the GC-ICP-MS with the other split ratios (Splitx) were calculated with the Eq. S1 where minitial represents the amount of volatile species contained in the initial calibration solution: = * 3.8:1 ( . 1) Absolute detection limits (ADL) were calculated according to Eq. S2, where μsignal and signal represent respectively the background signal and its standard deviation (SD) integrated before the retention time of each species and acal, the slope of each species expressed as peak height versus the amount injected.

Section C. Description of the PT system and determination of its parameters and correction factors
Each purging line of the PT system consists of a 500 mL PTFE purge bottle (Bohlender GmbH, Germany) connected to a manual PTFE flowmeter (Serie 4M, EM-Technik GmbH, Germany) delivering a N2 flow adjustable from 0.005 to 2 L.min -1 and measured using a digital N2 flowmeter (7000 flowmeter, Ellutia, Germany). The outlet of each purge bottle was connected to a U-shaped Vigreux column made of borosilicate glass (Schmizo, Switzerland) placed in an ice bath at -20°C made of sodium chloride (NaCl, 160 g.L -1 , technical grade, VWR) and crushed ice. Sorbent tubes were connected to the outlet of the Vigreux columns with PTFE union fittings (1/4"-1/4", Swagelok, Ohio). Prior to field campaigns, all tubing and parts of the PT system were washed overnight in 1 % HNO3 (Sigma Aldrich) and dried in a laminar flow hood.
In the laboratory, the recovery of the PT system for each volatile species was estimated by adding 1 µL of a working solution to a purge bottle filled with Milli-Q water and subsequently purging with N2 at different flow rates (250-500 mL.min -1 ) and for purging times ranging between 10 and 60 min. The PT recovery was assessed by dividing the peak areas of species recovered from the PT system by the peak areas of the same species directly loaded onto the same sorbent tube. The breakthrough (Eq. S3) for the PT system was estimated for various volume of N2 (2, 4, 6, 8, 10, 15, 20, 25 L; 400 mL.min -1 ) in Milli-Q water. Secondly, the influence of the salinity on the PT recovery was investigated using Milli-Q water that contained various NaCl concentrations (0-40 g.L -1 ), which was boiled for 1 hour to remove potential volatile species present, and purged at 400 ml.min -1 for 20 min. On-board R/V Svea, water samples (500 mL) were collected using 5-L Niskin bottles and the purge bottles were first rinsed with water from the Niskin bottle, then gently filled to the top using a silicone tubing placed at the bottom of the purge bottle to avoid degassing and closed with PTFE caps without headspace. Samples were either immediately purged or stored for maximum 1 h in the dark at 4 °C S8 before purging onto BM sorbent tubes with N2 (25 minutes at 400 mL.min -1 ). All the volatile species quantified in the aqueous samples were corrected according to the on-board PT recovery values determined every second day as follows: 1 µL of a working solution was added to a sample previously degassed with N2 (400 mL.min -1 ) for 2 hours and subsequentially purged for 25 minutes with a N2 flow set up at 400 mL.min -1 and compared to value obtained by adding 1 µL directly to a sorbent tube.
Potential contaminations during field campaigns were monitored using blank storage (n=5) and blank of the PT carrier gas (n=3; 25 min with 400 mL.min -1 of N2).

Section D. Estimation of breakthrough volumes and sampling procedure for atmospheric samples
Breakthrough volumes for BM sorbent tubes were determined by loading 1 µL of a working solution directly onto two sorbent tubes connected in series using the injection loop and subsequently flushing them with various volumes of either N2 (5 to 15 L) or the same volumes of ambient air (urban environment around Zurich with relative humidity ranging between 59 and 86%). The breakthrough (%) was determined based on the amount of volatile species detected in the first and second tubes according to Eq. S3.

S9
The automated MTS-32 was installed on the upper bridge of the RV (ca. 8 m above sea surface level), as far as possible and upwind from the ship exhaust. Every 12 hours, new sorbent tubes were placed into the autosampler of the MTS-32. All the sampled sorbent tubes were closed with brass caps, stored in double plastic zip bags at -20°C and analyzed within ten days after sampling.
Breakthrough for air and water samples was determined according to Eq. S3.  Table S6. Average trapping capacity (% ± SD) normalized to the highest intensity observed for each species loaded at 0.05 ng and 5 ng onto the six different sorbent tubes: air toxics (AT), graphitized carbon (GR), material emissions (ME), sulfur (SF), universal (UN) and biomonitoring (BM). The standard deviations (SD) are calculated from triplicate measurements.